1. Field of the Invention
The invention relates in general to a fabrication of semiconductor integrated circuits (ICs), and more particularly to a method of forming a complementary metal-oxide semiconductor (CMOS) sensor.
2. Description of the Related Art
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting light into an electrical signal that represents the intensity of the energy. CCD applications include monitors, transcription machines and cameras. Although CCDs have many strengths, CCDs use is restricted by their high cost and their volume. To reduce their cost, dimensions and energy consumption, a CMOS photo diode device has been develop. Because a CMOS photo diode device can be produced using conventional techniques, the cost and the volume of the sensor can be reduced. CMOS photo diode applications include PC cameras, digital cameras, etc.
A photo diode based on the theorem of a P-N junction can convert light into an electrical signal. Before energy in the form of photons strikes the photo diode, there is an electric field in the P-N junction. The electrons in N region do not diffuse towards P region and the holes in P region do not diffuse towards N region. When enough light strikes the photo diode, the light creates a number of electron-hole pairs. The electrons and the holes diffuse towards the P-N junction. When the electrons and the holes reach the P-N junction as a result of the effect of the inner electric field across the junction, the electrons flow to the N region and the holes flow to the P region. Thus a current is induced between the P-N junction electrodes. Ideally, a photo diode in the dark is an open-circuit. In other words there is no current induced by light while a photo diode is in the dark.
FIG. 1 is a schematic, cross-sectional view of a portion of a semiconductor device showing a conventional CMOS sensor. In FIG. 1, the conventional CMOS sensor includes a P-type substrate 100, a field oxide layer 104, a P-type well 110, a gate structure 120, an N-type source/drain region 122, an N-type sensor region 124, an depletion region 126, and a borophosphosilicate glass/silicon nitride glass dielectric layer 134.
When a light beam 140 passes through the depletion region 126 which works as a P-N junction, the depletion region 126 is excited and a number of electron-hole pairs are created. Thus the light is converted into an electric signal.
However, with respect to a CMOS image sensor, transmittance of light for the semiconductor structure used in a semiconductor image sensor is an important factor that seriously influences the quality of the image sensor. For example, it the imperative that the light transmittance is high enough. Only a high transmittance enables the light to arrive at the depletion region with a sufficiently high electric field in the semiconductor substrate. Upon arrival, the transmitted light induces electron-hole pairs due to excitation of photo-energy and thereby produces current in the intrinsic depletion region when light with varied wavelength transmits into the depletion region.
In general, the depletion region of a CMOS image sensor is formed far away from the surface of the semiconductor substrate. Since the wavelength of blue light, about 460 nanometers, is shorter than that of red light and green light, most of the blue light passing through the CMOS image sensor cannot arrive at the depletion region. The poor transmittance of the blue light causes the semiconductor substrate to receive insufficient light energy for current induction, leading to erroneous information.
Furthermore, a sensor region of a conventional CMOS image sensor is formed by implantation. The sensor region and the source/drain region of the CMOS image sensor are formed at the same implanting step so that the sensor region and the source/drain region have the same impurity varieties and the same implanting concentration. Arsenic (As) is usually doped into the substrate to form the source/drain region with a concentration of about 1.times.10.sup.15 atoms/cm.sup.2. As is heavier than phosphorous (P) and is doped into the substrate with a high energy of about 80 Kev so that the sensor region may be damaged from the high energy and the heavy atoms. The damage to the sensor region induces substrate leakage.